The present invention relates to a method of driving a liquid crystal display device which is equipped with thin film transistors (hereinafter referred to as TFTs) arranged in a matrix configuration and has a liquid crystal material of spontaneous polarization sandwiched between a cell array forming substrate (hereinafter referred to as an array substrate) and a color-filter substrate which are opposed to each other.
Conventionally, active-matrix liquid crystal display devices have been extensively used which are equipped as control devices with TFTs. This type of liquid crystal device has no crosstalk between respective pixels, provides high-contrast display, enables transmissive display, and has an advantage that the screen area can be increased easily.
In order to further improve such TFT liquid crystal display devices in response speed and viewing angle, investigations have recently been made on the use, as a liquid crystal material, of ferroelectric liquid crystal or antiferroelectric liquid crystal, which has spontaneous polarization like chiral smectic C phase liquid crystal or its sub-phase liquid crystal, in place of twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal). If, when the chiral smetic C phase liquid crystal or its sub-phase liquid crystal is driven with TFTs, the write time is shorter than the liquid crystal response time, then the holding voltage will lower because of a depolarization field (Hartmann: J. Appl. Phys. 66, 1132 (1989)).
The lowering of the holding voltage results in insufficient writing of data. When the effective applied voltage lowers, the contrast ratio decreases, which adversely affects image quality. The insufficient data writing can be compensated for by raising the applied voltage. To this end, however, medium voltage drivers or arrays must be used, which increases manufacturing cost. In addition, power consumption is increased. For this reason, particularly small, portable liquid crystal display devices that have low power consumption requirements cannot achieve a reduction of power consumption. Thus, the time that the battery can be used is reduced and the operability is degraded.
In frame inversion driving in which the applied voltage is driven in the positive/negative symmetric mode with its polarity inverted with each frame, when the absolute value of a signal voltage changes in some frame, the displayed image cannot immediately reach the quantity of transmitted light corresponding to a new signal voltage. In this case, the displayed image will reach the steady quantity of transmitted light while alternating between brightness and darkness over several frames. This phenomenon is called the step response (Verhulst et al: SID '94 digest, 337 (1994)), which degrades the image quality. To solve this problem, a method of eliminating the step response by driving the applied voltage in the asymmetric mode (as opposed to the symmetric mode) has also been investigated (Tanaka et al: SID '94 digest, 430 (1994)). In this case, although the contrast ratio is improved by accumulative response, there arise new problems of a decrease in the image response speed and image sticking due to ununiform distribution of impurities in liquid crystal or afterimage due to residual hysteresis.
Moreover, to solve the holding voltage and step response problems, means of increasing storage capacitance has also been investigated. In the case of liquid crystal display devices using TN liquid crystal, the storage capacitance is substantially the same as the capacitance associated with each pixel having liquid crystal. If, when ferroelectric or antiferroelectric liquid crystal is used, the storage capacitance is increased by a factor of ten or more over that in the TN liquid crystal display device, then the holding voltage problem will be solved. However, as long as the liquid crystal response speed is slow as it stands, the step response problem will not be solved. Furthermore, since increasing the storage capacitance results in an increase in power consumption, which makes the burden on the driving circuitry heavy. Thus, this method is not suited for small display devices.
As another method of solving the step response problem, conventionally a method has also been carried out in liquid crystal display devices that use TFTs or thin film diodes (hereinafter referred to as TFDs), which performs a reset operation of applying 0 volts to each pixel electrode immediately prior to a write operation to thereby make the holding voltage 0 volts for a portion of the time required to write in the applied voltage.
With the conventional method of performing a reset operation using a portion of the write time, however, the substantial write time becomes reduced unless the number of scanning lines is decreased because the frame time is predetermined. A decrease in the write time results in insufficient writing in. Thus, a sufficient improvement in contrast cannot be achieved. With liquid crystal display devices in which the write time is reduced as a result of increasing the number of lines to achieve high definition or increase the screen size, the write time becomes further reduced due to the reset operation. The bad effects of poor writing are enhanced and the display quality is degraded considerably due to poor contrast.
A liquid crystal display device has also been proposed which uses a circuit arrangement that is equipped with two TFDs for each pixel and uses two signal lines for data and reference, the data signal line being used to write data into a line and the reference signal line being used to reset other lines than the line that is being written into by the data signal line (Verhulst et al: IDRC '94 digest, 377 (1994)). With this circuit arrangement, the numbers of switching devices and lines per pixel increase and the driving waveforms become complicated, decreasing the manufacturing yield and preventing manufacturing cost from lowering. Further, the use of TFDs makes it difficult to control variations in overall liquid crystal display device characteristics.